1. Field of the Invention
The present invention relates to a depth sensor element, especially to a complementary metal-oxide-semiconductor (CMOS) depth sensor element.
2. Description of the Prior Arts
A conventional CMOS depth sensor element of an active image sensor as shown in FIG. 6 comprises a sensing element 50 and a reading unit 60. The sensing unit 50 comprises a photogate element 70. The reading unit 60 comprises a transfer transistor Qtx, a reset transistor Q1, an amplifier transistor Q2 and a selection transistor Q3. The photogate element 70 shown in FIG. 6 is a n-type photogate element. A p-type epitaxy 52 is formed on a p-type substrate 51. Then a n-type doped well 53 is formed on the p-type epitaxy 52 corresponding to a photosensitive region A. Then a photogate 71 is formed on the n-type doped well 53. The photogate 71 comprises an insulating layer 711 and a poly-silicon layer 712.
The transfer transistor Qtx is a n-type complementary metal-oxide-semiconductor (NMOS) transistor. The transfer gate GO of the transfer transistor Qtx is formed on the p-type epitaxy 52 of the p-type substrate 51. The a first n+ doped region 531 a and a second n+ doped region 53 lb are formed in the p-type epitaxy 52 and respectively correspond to under two sides of the transfer gate G0. The first n+ doped region 531a is formed on one side of the n-type doped well 53. The transfer gate G0 of the transfer transistor Qtx and the photogate 71 of the photogate element are conductively connected to each other at the bottom. The second n+ doped region 531b functions as a transmitting node FD of the conventional depth sensor element and is conductively connected to the reset transistor Q1 and the amplifier transistor Q2.
The reset transistor Q1, the amplifier transistor Q2 and the selection transistor Q3 as shown in FIG. 6 are all NMOS transistors and are represented in electronic element symbol. The source S1 of the reset transistor Q1 is connected to the transmitting node FD. The drain D1 of the reset transistor Q1 is connected to a high potential voltage Vcc. The gate G2 of the amplifier transistor Q2 is also connected to the transmitting node FD. The drain D2 of the amplifier transistor Q2 is also connected to the high potential voltage Vcc. The source S2 of the amplifier transistor Q2 is connected to the drain D3 of the selection transistor Q3. The gate G3 of the selection transistor Q3 is connected to a corresponding one of the row selection line Yj of the image sensor. The source S3 of the selection transistor Q3 is connected to a corresponding one of the column bit line Xi.
When light is emitted to the photosensitive region A, the photogate element 70 is excited to generate majority carriers. For example, for the n-type photogate element 70, the majority carriers are electrons. Then a driving signal TX is output to the transfer gate GO of the transfer transistor Qtx for generating a channel between the first and second n+ doped region 531a, 531b. The majority carriers generated from the photogate element 70 excited by light diffuse to the first n+ doped region 531a and then moves to the second n+ doped region 531b through the channel. Therefore, the majority carriers are collected in the transmitting node FD. Then, the reset transistor Q1 and the amplifier transistor Q2 are controlled to amplify a sensing signal corresponding to the majority carriers of the transmitting node FD. When a selection signal is transmitted to the row selection line Yj connecting to the gate G3 of the selection transistor Q3, the selection transistor Q3 is switched on to transmit the amplified sensing signal to the corresponding column bit line Xi.
In conclusion, when the channel of the transfer transistor Qtx is generated, the majority carriers generated from the photogate element 70 excited by light are collected to the transmitting node FD. However, a PN junction is between the first n+ doped region 531a and the p-type epitaxy 52 so that a gate voltage must be large enough to generate the channel for transferring the majority carriers of the photogate element 70 when the transfer transistor Qtx is switched on. Further, the majority carriers are transferred to the first n+ doped region 531a by diffusion so that the transferring speed of the majority carriers are slow. Therefore, the semiconductor structure of the conventional depth sensor element is against the development of the image sensor with high speed and needs to be further improved.